1. Field of the Invention
The present invention relates to deposition of a diamond layer on a substrate by means of a Penning-type plasma discharge.
2. Description of the Related Art
Electronic properties of diamond layers or films, coupled with their high thermal conductivity, make the films extremely attractive for semiconductors. High-power transistors that require no external heat sinks are one possibility for single-crystal films. High-density integrated circuits could be packed close enough to shorten electron paths and speed up the circuits. Diamond's resistance to radiation makes these ICs attractive for use in varied applications. Diamond is superior even to gallium arsenide in virtually all electrical properties, and can be operated at elevated temperatures on the order of 700.degree. C.
Diamond film layers may find beneficial applications in a number of other areas, including infrared transparent windows for sensors, dome coatings for abrasion resistance, heat sinking (carbon has five times the thermal conductance of copper) for microelectronics and other applications, and hard coatings for tools, high-wear parts, and similar tribological applications.
Diamond has been deposited by a wide number of processes, such as beam-assisted chemical vapor deposition (CVD) and carbon-ion impact from a carbon ion beam.
A CVD process starts by introducing a mixture of hydrogen and a hydrocarbon gas, usually methane, into a low pressure chamber. The mixture is heated to about 2000.degree. C. using an electric filament, microwave, or other heat source. At this temperature, the hydrogen and methane dissociate into hydrogen and carbon atoms. The carbon atoms are deposited on the substrate, which is heated to 600.degree. to 1000.degree. C.
The hydrogen enhances the formation of the diamond film using CVD by inhibiting graphitization. In a diamond, each carbon atom has four bonds to other carbon atoms. If not enough carbon atoms are present to form four bonds, atoms will form two bonds, creating a graphite-type surface. Hydrogen atoms hook up with unattached bonds of carbon atoms and stabilize them until other carbon atoms come along to take their place. Each incoming carbon atom forms a single bond with each of four neighbors, resulting in the diamond structure. Most of the hydrogen atoms are released.
Representative examples of beam-assisted CVD processes are found in "Ion beam sputter-deposited diamondlike films", by B. A. Banks et al, J. Vac. Sci. Technol., 21(3), Sept./Oct. 1982, pp. 807-814; "Large Area Chemical Vapour Deposition of Diamond Particles and Films Using Magneto-Microwave Plasma", by H. Kawarada et al, Japanese Journal of Applied Physics, Vol. 26, no. 6, June 1987, pp. L1032-L1034; and "Synthesis of diamond films in a rf induction thermal plasma", by S. Matsumoto et al, Appl. Phys. Lett. 51(10), Sep. 7, 1987, pp. 737-739.
The carbon-ion-beam method is the closest known prior art, in that pure carbon is used, but the carbon ion beams typically have either inappropriately high energies, or are very low in current density or fluence due to space-charge limitations in the beam forming optics. The high energies cause dislocations, twinning, and other defects. Low fluences result in exceedingly slow growth rates. In both cases, the substrate must be heated, with multiple attendant problems. A treatise on this method of diamond film deposition is found in "The epitaxial synthesis of diamond by the deposition of low energy carbon ions", by J. H. Freeman et al, Vacuum/volume 34/numbers 1-2/pages 305-314/1984 (Great Britain). All of these processes require the substrate to be heated to temperatures around 600.degree. C. When the substrate cools, differential expansion places the diamond film in stress, limiting the thickness and quality of the films. Few processes produce single crystals of more than a few micrometers in size, with polycrystalline or amorphous materials being most commonly produced.